1. Technical Field
The present invention relates to a projection lens system for projecting display information and the like from a light valve such as a liquid crystal display device in an enlarged manner. Particularly, the invention relates to a projection lens system suitable for the front projection type display apparatus and a projection type display apparatus using the same.
2. Related Art
Recently, projection type display apparatuses which use light valves such as a liquid crystal display device and a DMD display device have become widespread. In particular, one such widely used apparatus is a projection type display apparatus having a configuration in which three light valves corresponding to illuminating rays of RGB three primary colors are employed, the rays modulated by the respective light valves are synthesized by a prism or the like, and an image is displayed on a screen through a projection lens.
In the projection lens system provided in the projection type display apparatus that synthesizes the rays, which are modulated by the three light valves, through a color synthesizing optical system and projects the rays, a long back focal length is required in order to dispose a prism for synthesizing the colors as described above and the like, and in order to avoid a thermal problem. Furthermore, in the color synthesizing optical system, a spectral characteristic varies with the angle of incident light. Hence, it is required for the projection lens system to have a characteristic in which an entrance pupil is sufficiently far away in view from the reduction side, that is, telecentricity. Further, it is also required for the system to be a fast lens and to correct aberrations in accordance with resolutions of the light valves.
The projection lens systems satisfying some of the above-mentioned requirements include, for example, the lens systems disclosed in the following Patent Document 1 (JP-A-2003-015033 corresponding to US-A-2003/0137744) and Patent Document 2 (JP-A-2004-326079 corresponding to US-A-2004/0257666). Further, applicants of the invention also filed patent applications relating to such projection lens systems (see the following Patent Document 3 (JP-A-2008-309988)).
All the projection lens systems disclosed in these Patent Documents are formed as wide-angle lens systems of which each angle of view is 100 degrees or more.
By the way, as the projection type display apparatuses, a front type apparatus and a rear type apparatus has been developed. In the front type apparatus, the projection lens system is disposed on the same side as an observer relative to a screen and light emitted from the projection lens system forms an image on the reflective type screen. In the rear type apparatus, the projection lens system and the observer are disposed with the screen interposed therebetween and light emitted from the projection lens system forms an image on the transmissive type screen.
As an example of the rear projection type display apparatus, a rear projection television includes components from the light source to the screen to be incorporated in a cabinet and light containing image information from the projection lens system disposed on the rear side of the cabinet is projected toward the screen disposed on the front side of the cabinet. In Patent Documents 1 to 3, it is also assumed that the projection lens system is provided in such a rear projection type display apparatus.
However, in the projection lens system of the rear type apparatus, a prism or a mirror for deflecting the optical axis is disposed in order to decrease a thickness of the cabinet, and the entire length of the lens system is elongated along the optical axis. Hence, the spatial size of the apparatus increases in the end. Accordingly, when the projection lens system described in the patent publications is used in the front projection type display apparatus, a problem arises in that the apparatus can not be downsized.
Further, in the front projection type display system (including the display apparatus and the screen), the area of a space for installing the system is often restricted as compared with the rear type. From this viewpoint, the working distance from the display apparatus to the screen is required to be decreased, and the angle of view of the projection lens system is required to be increased.